1. Field of the Invention
The present invention relates to a non-contact temperature sensor and a detecting circuit therefor, and more particularly to a non-contact temperature sensor for sensing or detecting the surface temperature of a rolling body such as a heating/fixing roller of a fixing device in order to fix a non-fixed toner image on a sheet of paper in a fixing device such as a copying machine.
2. Description of the Related Art
Conventionally, a contact temperature sensor has been mainly used as a temperature sensor for a heating/fixing roller of a copier. The contact temperature sensor detects the temperature of the roller in a manner of keeping a heat-sensitive element in contact with the surface of the roller. The contact temperature sensor has an advantage that it can precisely detect the surface temperature of the heating/fixing roller, but also has a disadvantage that since a portion of the heat-sensitive element is kept in pressure-contact with on the surface of the heating/fixing roller under certain pressure, the surface of the roller may be damaged by a contact member or a heat-sensitive element.
Further, the contact member of the contact temperature sensor must be made of a material having a high abrasion resistance which can withstand the use for a long time and having a certain thickness to resist the pressure of a spring material for attaching the heat-sensitive element. Therefore, the contact temperature sensor has a disadvantage of giving great heat capacity to the heat-sensitive element, and not capable of providing a desired thermal response characteristic.
In order to overcome these disadvantages, several proposals of a non-contact sensor have been made.
The first proposal is a temperature sensor proposed in J-UM-4-122341 (Laid-Open) by the inventors of the present invention. This temperature sensor includes a heat-resistive insulating base plate having two leads of a conductive foil formed thereon, a heat-sensitive element located in a small hole between these two leads and connected therebetween, a supporting body which has at least a large opening for the heat-sensitive element and for the vicinity of the leads of the heat-resisting plate and attaches the heat-resisting plate on the lower face of the opening, and a thin film shielding plate having smaller thermal capacity than that of a supporting member attached on the upper face of the opening. This non-contact temperature sensor, which can sense the temperature of a body to be detected in a non-contact manner and reduce the thermal capacity of the sensor, can provide an excellent thermal response characteristic.
The second proposal is a non-contact temperature sensor as shown in FIG. 13. This non-contact temperature sensor includes a cylindrical non-contact temperature sensor body 10, an optical system 3 of a Fresnel lens provided at the tip of the sensor body 10, an infrared ray detecting element 1 of a thermo-pile, a temperature sensor 2 for measuring the temperature of the infrared ray detecting element 1 and another temperature sensor 4 for measuring the temperature of the optical system 3. The detecting element 1 and temperature sensors 2 and 4 are housed in the sensor body 1. The temperature sensors 2 and 4 may be posisters.
FIG. 14 shows a signal processing circuit of the non-contact temperature sensor shown in FIG. 13. In operation, an output from the infrared detecting element 1 is amplified by an amplifying unit 6 via a polarity-exchanging unit 5. The amplified output from the amplifying unit 6 is A/D converted into a digital signal by an A/D converting unit 9. The digital signal is further processed by a microcomputer 11. The temperature sensors 2 and 4 are supplied with a constant current from a constant current source 8. The signal from each of the temperature sensors 2 and 4, is switched by a switching unit 7, and converted into a digital signal by the A/D converting unit 7, which is in turn processed by the microcomputer 11. The non-contact temperature sensor is provided with a temperature compensating means for compensating for an error due to the optical system on the basis of a temperature difference between the temperatures sensed by the temperature sensors 2 and 4.
The non-contact temperature sensor according to the first proposal, where it is used for the heating/fixing roller of a copying machine, has a disadvantage that it can sense the temperature with sufficient sensitivity only when a distance between the heat-sensitive surface of the heat-sensitive element on the heat resisting plate and the roller face is set at about 0.5 mm. However, it is very difficult to set such a small distance. In addition, where the non-contact temperature sensor is mounted in proximity to the heating/fixing roller, it may be broken owing to paper jam of the copier which may occur.
The non-contact temperature sensor according to the second proposal shown in FIG. 13, in which the optical system is provided on the infrared ray incident face, has a disadvantage that toners are likely to be deposited on the optical system 3 when it is mounted on the heating/fixing roller, polluting the surface thereof, and that the transmitting amount of the infrared rays may be changed owing to the toners deposited on the surface of the optical system. As the pollution of the surface of the optical system is a result of the deposition of the toners or dust, the detecting sensitivity is lowered so that the temperature cannot be detected precisely. Therefore, where precise control of the temperature is required, this temperature sensor cannot be used.
The infrared rays radiated from the body to be detected is influenced by the ambient temperature. Therefore, the non-contact temperature sensor is provided with another separate temperature sensor for measuring the ambient temperature to measure a change in the ambient temperature. In this case, using a microcomputer 11 in the signal processing circuit as shown in FIG. 14, a complicated numerical computation of changing a function table must be carried out in accordance with changes in the transmission of the infrared rays and in the ambient temperature.
A thermistor, bolometer, a thermopile, pyroelectric type sensor may be used as an infrared ray detecting element. However, the thermistor and thermopile are low in sensitivity and high in cost. The pyroelectric type sensor, which requires a chopper, is problematic in reliability. Because of a technical problem of temperature compensation, the pyroelectric type sensor is also difficult to be used at a high temperature where a fixing device is used.
An object of the present invention is to provide a non-contact temperature sensor and detecting circuit used therefor which can detect the surface temperature of a body to-be-detected precisely for a short time.
In order to attain the above object, in accordance with the present invention, there is provided a non-contact temperature sensor comprising a holder serving as a light conducting portion for guiding infrared rays incident from a first opening at its one end; a plastic film arranged at a second opening of the other end of said light conducting portion; a cover for sealing the second opening at said other end of the holder so as to provide a space between itself and said plastic film, an infrared ray detecting heat-sensitive element arranged on the side of said space of said plastic film, for detecting the infrared rays incident from said first opening; and a temperature compensating heat-sensitive means arranged in the vicinity of the holder, for detecting the temperature of said holder.
In the non-contact temperature sensor according to the present invention, the infrared rays radiated from the surface of a body-to-be-detected, e.g. a heating/fixing roller are incident on the first opening of the holder and guided to the plastic film through the light conducting portion. Then, the plastic film absorbs the infrared rays at a wavelength corresponding to its inherent absorbing spectrum so that its temperature rises. The infrared ray detecting element detects a temperature rise to detect the surface temperature of the body. The temperature compensating means detects a temperature change in the holder and also the temperature change due to thermal radiation or convection around the sensor, thereby compensating for the surface temperature of the body detected by the infrared ray detecting element.
In the above configuration of the non-contact temperature sensor according to the present invention, if the first opening from which the infrared rays are incident is formed in a horizontally lengthy shape, when the opening is aligned with the direction of a heat generating body which is lengthy in an axial direction, such as a heating/fixing roller, the sensor can effectively trap the heat radiated from the body-to-be-detected, thereby enhancing the detecting sensitivity. In addition, since the space serves as a thermal insulating layer, if the infrared rays and the ambient temperature are detected, the temperature of the body can be detected precisely.
Preferably, the internal wall of the light conducting portion of the non-contact temperature sensor constitutes a reflecting face capable of reflecting the infrared rays. Therefore, the radiated heat can be effectively trapped by the infrared ray detecting element to improve the detecting sensitivity.
Since the two temperature compensating elements preferably arranged in proximity to each other are thermally coupled, the output from the infrared ray detecting element can be made free from that influence by the ambient temperature. Therefore, the structure of the detection circuit for the non-contact sensor can be simplified.
The plastic film of the heat-sensitive portion is preferably made of an infrared ray absorbing material. This improves the detecting sensitivity of the non-contact temperature sensor.
A detection circuit for a non-contact temperature sensor includes a first series circuit composed of said infrared ray detecting element and said first temperature compensating element, a second series circuit composed of said second temperature compensating element and a resistor element; and an arithmetic unit for processing an output from said first series circuit and another output from said second circuit to detect a surface temperature of a body-to-be-detected.
In this configuration, the ambient temperature can be canceled so that the surface temperature of the body-to-be-detected can be detected precisely.
The infrared ray detecting element is placed on the space behind the plastic film and is not exposed to the ambient atmosphere. Therefore, the infrared ray detecting element is not polluted so that the temperature of the body-to-be detected can be detected stably for a long time.
The above and other objects and features of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings.